CustomChocolateShop / Overcharging on shipping

I can't complain about the product or service or speed of it. That all was simply great. My concern is with the amount i am being charged for shipping.
Last weekend, in error, i ordered #4s of a particular item. When i received it thru USPS i realized it was too small and i needed 5s. I returned the box with 4s and paid only $17 and cents for shipping at my nearest post office.
Today, while ordering the 5s i'm finding out that the company charge for shipping is $29 or roughly $12 x 2 = $24 MORE for their shipments of first the 4s and now the 5s.
Is this common practice?
Am i being overcharged? Is this a conceiled added profit for this company? Or is it deceit? Robbery?